Lowering the exhaust gas emissions of diesel combustion engines is one of the main goals of engine development. This improvement can be achieved in several ways. Firstly, the exhaust gas components are reduced by optimizing the in-engine combustion. Secondly, polluting and toxicological exhaust components can be minimized by means of exhaust gas aftertreatment systems of emissions. The recirculation of exhaust gas into the fresh air mass flow (exhaust gas recirculation, EGR) is a constructive measure for reduction of nitrogen oxide emissions in diesel engines. In this exhaust gas recirculation system, the EGR valve is a key component and often combined with an EGR cooler. Depending on the concept of the engine there can be high or low pressure exhaust gas recirculation, i.e. the exhaust gas is removed either before or after the turbocharger, or there can be a combination of both systems.
In the present study, the interactions and influences of organic deposits in turbocharged diesel engines are considered. The examined Volkswagen engine is installed in vehicles to comply with the exhaust emission level Euro 5, and is equipped with a water-cooled high-pressure EGR and an exhaust aftertreatment system. Therefore, the focus of this study is on the subarea of the deposits in high pressure EGR systems.
The exhaust gas from the high-pressure EGR is directed back into the fresh air intake system before it reaches the turbocharger. The extracted gas is not purified by exhaust gas aftertreatment systems. Therefore, all exhaust gas components are guided through the EGR cooler and through the EGR valve. This can lead to contamination that could potentially affect the cooling capacity, the recirculated exhaust gas mass and the combustion process. In this thesis, the operation of the mentioned exhaust gas recirculation and the various combustion processes in a diesel engine are described in detail.
Three aspects of the dirt or deposits in the EGR system are investigated more closely.
First aspect: The physical and chemical deposition mechanisms, which can lead to contamination and deposits in EGR systems, are pointed out.
Second aspect: The deposits are analyzed for their chemical composition. It could be shown that the deposits found are completely of organic chemical nature and consist mainly of diesel soot, polycyclic aromatic hydrocarbons, unburned fuel, and various polymeric structures. The latter is formed in situ by the prevailing temperatures and the present molecular combinations.
Third aspect: The influence factors and mechanisms of deposit formation are described and simulated in engine test rigs and laboratory tests. In particular, the impact of polycyclic aromatic hydrocarbons and the buildup of organic polymers, which are based on phenol-aldehyde resins, have been found to be significant mechanisms.
Furthermore, network formation mechanisms with biodiesel molecules and esterification reactions have crucial impact on formation of deposits.
Moreover, a strong influence of the exhaust and cooler temperatures are detected. The greater the difference between cooling water temperature and exhaust gas temperature, the better the chemical components are deposited on the cooler surface. Furthermore, exhaust temperatures up to 800 Grad C reinforce the pyrolysis reaction, and the formation of networks of polycyclic aromatic hydrocarbons. In conjunction with subsequent cooling down, the condensed exhaust gas components are cooled and the network formation reaction is interrupted and is kept in that state. In combination with diesel soot particles, a surface is created, which is able to accommodate the additional exhaust components. The subsequent high-temperature phases pyrolyze the organic material, and thereby produce a solid and adherent surface or insulation layer. The heat contained in the exhaust gas can no longer be dissipated via the heat exchanger. Also, the pyrolysis reactions are enhanced by increasing temperatures at the surface. As a result the deposit formation catalyzes itself.